1. Field of the Invention
The present invention relates to crop breeding, and more specifically to methods for breeding new crop cultivars. The methods can be used to develop genetically-diverse selection populations by recurrent crossing of a donor line and its derivatives to one or more hybrid parents.
2. Description of Related Art
Breeders are continually developing new “line” cultivars through various plant genetic improvement programs. These cultivars are generally pureline cultivars of predominantly self-fertilized species (e.g., bean, pea, lettuce, wheat, etc.) as well as inbred line parents to produce hybrid and synthetic cultivars of both predominantly self-fertilized and cross-fertilized species (e.g., tomato, pepper, corn, sorghum, onion, carrot, etc.).
Efficient development of new line cultivars depends on combining favorable alleles for one or more important traits of interest with groups of genes that together impart outstanding field performance such as adaptation to prevailing environments, yield, and preferred horticultural and agronomic traits. Effective selection for new combinations of favorable alleles and performance traits depends on creating diverse selection populations that are segregating for genes controlling traits of interest.
Development and use of genetically-diverse selection (breeding) populations are vital to the success of a breeding program, and rightly deserves considerable attention. While investigating the optimal number and size of breeding populations for inbred line development, Bernardo (2003) made the following statement; “New inbreds are most often developed from crosses among elite inbreds in cultivar development programs (Allard 1960, p. 282). Specifically, two inbreds are first selected as parents of an F2 or backcross breeding population. New inbreds are then developed by pedigree selection, single-seed descent, or the bulk method of breeding. A breeder typically creates, selfs, and selects in several breeding populations at a time. This scheme (which has become known as advanced cycle breeding) for developing new inbreds is widely used both in self-pollinated crops such as soybean (Glycine max (L.) Merrill; Hartwig 1973) and wheat (Triticum aestivum L.; Heyne and Smith 1967), and in hybrid crops, such as maize (Zea mays L.; Hallauer) 1990).”
“Advanced Cycle Breeding” may be followed when the performance of an inbred parent is known and that parent is available for making new cross combinations. Other sources of useful genetic variability are elite commercial hybrids of which the parental inbreds are not commonly known or available, but which are accessible for use as a source of new gametes because they are public hybrids or registered with a Plant Variety Protection Certificate that expressly allows for such use in breeding.
When commercial F1 hybrids are available, a common approach to evaluating and using them as a source of new inbred lines is to “self-down” the hybrid, select for important traits and evaluate new potential inbred parents for combining ability. This limits the genetic variability in each population to the contributions of only the inbreds that are the hybrid parents, e.g., two for a single cross, three for a 3-way, and four for a double-cross hybrid. An additional short-coming is the limited effective recombination that occurs when inbreeding is intense, i.e., with self-fertilization at each generation and even more-so when inbreds are produced from doubled haploids.
In contrast to “selfing-down”, recurrent crossing to one or more hybrid (recurrent) parent(s) assures that effective recombination continues at each “backcross”, whether to the same or different hybrid parent(s). The selection units chosen after each cycle of recurrent crossing can be used either for additional recurrent crossing or selected for traits of interest and performance traits including combining ability during or at the end of each generation of inbreeding.
“The potential advantage of mating genetically diverse parents is that each may contribute unique alleles, which when combined together may result in a superior individual” (Fehr, 1987). The theoretical and practical challenge is to create a selection population that has broad genetic diversity concurrent with high mean performance. Intermating parents which have one or a few elite alleles for a single trait but are otherwise less-adapted may increase the genetic diversity but also lower the mean trait phenotypic value of the selection population. On the other hand, intermating parents that are well-adapted and higher performing will likely produce a population with high mean phenotypic value, but less genetic variability. An attractive alternative approach is to use a method that allows for ongoing recombination of DNA during sexual reproduction to generate genetic variability while minimally reducing the mean trait phenotypic value of the selection population.
Typically the recurrent parent used in backcross breeding is one well-defined parent i.e., “The recurrent parent in a breeding program should be a highly acceptable genotype, except for the trait that will be altered by backcrossing. The general principle is that the genotype obtained from backcrossing will not be improved for any character except the one being transferred from the donor parent” (Fehr, 1987, p. 361).
In the standard backcross method, a recurrent parent is intended to be an inbred line that is homogeneous or nearly so rather than a hybrid. “Additionally, it should be recognized that the recurrent parent is not composed of a single pure line but is likely to be made up of many closely related pure lines” (Allard, 1960, p. 155). Further he states, “After the third backcross, however, the population usually resembles the recurrent parent so closely that selection on an individual-plant basis is largely ineffective except for the character being transferred.” Clearly, when a hybrid line is used as a parent for recurrent crossing, this would not be the case.
Allard (1960, p. 151) states that, “If a backcross program is to produce a successful variety, the following three requirements must be satisfied: (1) a satisfactory recurrent parent must exist; (2) it must be possible to retain a worthwhile intensity of the character under transfer through several backcrosses; and (3) sufficient backcrosses must be used to reconstitute the recurrent parent to a high degree.” Again, if a hybrid line is used as the hybrid parent during recurrent crossing, conclusions reached in (3) above would not be the case.